1. Field of the Invention
The present invention relates to a low-vacuum scanning electron microscope that can maintain the interior of a specimen chamber at a low vacuum, can prevent charging of a specimen, and permits observation of a specimen in its intact state even if it contains moisture.
2. Description of Related Art
In scanning electron microscopy, an electron beam is sharply focused onto a specimen, and a desired area on the specimen is scanned with the electron beam. As the electron beam hits the specimen, secondary electrons and backscattered electrons are ejected. These electrons are detected, and the resulting detection signal is supplied to a cathode-ray tube synchronized with the scanning of the primary electron beam. In this way, a scanned image of the specimen is displayed.
The essential structure of this scanning electron microscope is described by referring to FIG. 1. An electron gun (not shown) for emitting and accelerating an electron beam, condenser lenses (not shown), and other components are mounted above an objective lens 1. The primary electron beam 2 is focused sharply by the condenser lenses and objective lens 1 and directed at a specimen 3. Scan coils (not shown) for scanning the primary electron beam 2 over the specimen in two directions are disposed above or inside the objective lens 1.
As the electron beam 2 hits the specimen 3, secondary electrons are produced. These secondary electrons are collected by a mesh-like collector 5. A voltage of about 300 V is applied to the collector from a power supply 4. The secondary electrons collected by the collector 5 are guided to a secondary electron detector 7. This detector 7 is composed of a corona ring 9 applied with a high voltage of about 10 kV from a corona ring power supply 8, a scintillator 10 on which secondary electrons accelerated by the corona ring 9 impinge, and a photomultiplier tube 11 for converting light from the scintillator into an electrical signal.
The secondary electron signal from the secondary electron detector 7 is supplied as a brightness-modulating signal to a CRT via an amplifier (not shown) and via a signal-processing circuit (not shown) for adjusting the brightness and the contrast. The CRT is synchronized with the scanning of the primary electron beam. As a result, a secondary electron (SE) scanned image of a certain two-dimensional area on the specimen is displayed on the viewing screen of the CRT.
Where the specimen 3 is an insulator or contains moisture, it is customary to set low the degree of vacuum in the specimen chamber where the specimen 3 is placed. However, if the degree of vacuum inside the specimen chamber is lowered, electric discharging occurs because a high voltage is applied to the corona ring 9. Therefore, the secondary electron detector 7 cannot be used in a low vacuum. Consequently, under a low-vacuum condition, a backscattered electron detector to which high voltage is not applied is normally used to perform backscattered electron imaging.
Although a backscattered electron image contains a large amount of information about the composition of the specimen, the amount of information regarding the topography of the surface is small compared with a secondary electron image. For this reason, it has been difficult to image the three-dimensional topography of the specimen surface.
Accordingly, an absorption current detection method can be used as a method of obtaining an image similar to a secondary electron image even at a low vacuum. As a primary electron beam strikes a specimen, secondary electrons and backscattered electrons are ejected from the specimen. A feeble absorption current flowing through the specimen is measured. An image is created from variations in the magnitude of the current.
FIG. 2 shows a specific structure for implementing this detection method. An electrical current absorbed into the specimen 3 is measured by a specimen absorption current-measuring instrument 15. The measured absorption current is amplified by a current amplifier circuit 16 and is supplied to the CRT (not shown). Since the absorption current is far weaker than the incident current, introduction of noise is unavoidable. Therefore, it is difficult to obtain an image at a higher resolution unless the incident current is increased extremely.
It is an object of the present invention to provide a low-vacuum scanning electron microscope capable of efficiently obtaining an image similar to a secondary electron image even at a low degree of vacuum.
A low-vacuum scanning electron microscope, according to a first embodiment of the present invention, comprises: an electron gun for producing and accelerating an electron beam; condenser lenses for focusing the electron beam onto a specimen placed within a specimen chamber in a low vacuum, the specimen being maintained at a potential; scanning means for scanning the electron beam over the specimen in two dimensions; voltage generation means for setting the potential of the specimen such that the specimen is at a negative potential with respect to the specimen chamber; detection means for detecting an electric current flowing through the specimen and/or an electric current flowing through a specimen holder holding the specimen thereon; and means for obtaining a scanned image of the specimen based on an output signal from the current detection means. The voltage generation means sets the potential of the specimen based on information about the pressure inside the specimen chamber and on information about the distance between the specimen and the condenser lenses.
A low-pressure scanning electron microscope, in accordance with a second embodiment of the present invention, comprises: an electron gun for producing and accelerating an electron beam; condenser lenses for focusing the electron beam onto a specimen placed within a specimen chamber in a low vacuum; scanning means for scanning the electron beam over the specimen in two dimensions; an electrode placed above the specimen; voltage generation means for applying a voltage between the specimen and the electrode to accelerate secondary electrons toward the electrode, the secondary electrons being emitted from the specimen by electron beam irradiation; current detection means for detecting an electric current flowing through the specimen and/or an electric current flowing through a specimen holder holding the specimen thereon; and means for obtaining a scanned image of the specimen based on an output signal from the current detection means. The voltage generation means controls the voltage applied between the specimen and the electrode based on information about the pressure inside the specimen chamber and on information about the distance between the specimen and the condenser lenses.
Other objects and features of the invention will appear in the course of the description thereof, which follows.